A steam generator of the type often used for a pressurized-water reactor coolant loop, is disclosed by the U.S. Webster Pat. No. 3,766,892, dated Oct. 26, 1973. Such a steam generator has a vertical casing with its bottom closed by a tube plate in which the bottom ends of the hot and cold legs of a U-tube bundle heat exchanger, are mounted, the U-tube bundle being encircled by a shroud having a bottom end spaced above the tube plate and a top mounting water separators. The casing has one or more feed-water inlets and is kept filled with water held at a level such that the water thermally rises within the shroud while vaporizing, flows over the top of the shroud, possibly being water separated from the rising vapor, and flows down a descent space formed between the shroud and the inside of the casing, to then flow inwardly over the tube plate and rise again, thus maintaining a circulation.
The type disclosed by the Webster patent has a preheater formed by a vertical wall extending through the corridor formed between the two legs, and connecting with the inside of the shroud, the shroud being substantially cylindrical and forming a substantially semicylindrical side for the preheater. A horizontal partition divides the preheater into upper and lower sections, the generator's casing having two feed-water inlets respectively connected individually with the two sections at a position adjacent to the partition, both the top and bottom of the preheater having outlets so that incoming feed-water flows after preheating, are discharged downwardly towards the tube plate and upwardly in the direction of the top of the shroud. Baffles in both sections provide sinuous water flows wiping heat from the tubes of the cold leg.
The two feed-water inlets connect with the bottom and top, respectively, of the upper and lower preheater sections, by what is in effect a single arcuate manifold which is itself separated into upper and lower sections by an arcuate horizontal partition into upper and lower sections, the two manifolds feeding through an arcuate series of holes formed through the shroud in each instance and positioned, in each instance, adjacent to the preheater's partition. The preheater and manifold partitions are necessarily positioned in the same or approximately the same planes, and they may be formed by a single appropriately shaped flat plate extending from the preheater and through the shroud and into the manifold.
It can be seen that with the reactor coolant continually circulating through the U-tube bundle, that heat is removed from the cold leg by upwardly and downwardly directed flows of incoming feed water, the water, while in the preheater and flowing, remaining at temperatures which under the steam pressure existing in the generator, avoid boiling of the water. It is desirable to avoid such boiling because it results in operational instabilities, such oscillations in the water circulating in the generator, and, boiling within the preheater throws down concentrations of corrosive products inevitably contained by the feed water.
When the demand for steam produced by this type of steam generator drops or terminates for any reason, and the generator operates under low-load conditions as compared to its normal loading, the demand for the input of feed water correspondingly drops or is eliminated. Under such conditions, it may be necessary to entirely stop the flow of feed water through the two feed-water input connections, and to provide through another and smaller input connection, just enough feed water to maintain the water level in the steam generator as required to maintain the described circulation up within the shroud and down outside the shroud through the descent space between the shroud and casing of the generator.
However, with the normal feed water to the two preheater sections, then terminated, there is no circulation through either section of the preheater and, therefore, the described boiling within the preheater occurs or can occur. The partitions in the preheater and its manifold block thermal upwardly flow through the preheater inside of the shroud.
Now it can be seen that the practical operation of the described type of steam generator, presents a problem when the generator must be operated under low-load conditions.